Entries from April 2014 ↓

Bazooka Dipole

Some two or three decades ago I built a bazooka dipole for 40 meters. I chose 40 meters because I had just enough old RG8 coax to do the job.

The antenna worked fine and the coax was heavy enough to serve as two of the guys for the push-up mast I was using for my 15 meter quad.

Time marches on and the bazooka fell into disrepair and was abandoned in favor of a 40 meter loop.

Recently I took another look at the bazooka design and finally built one for 40 meters out of RG8X coax. Again, I chose 40 meters because I had just enough coax to do the job. The coaxial center section was 53 feet, 8 inches long and the insulated wire ends extended another 4.5 feet on each leg. The end result was 1.2:1 SWR bandwidth from 7.000 to 7.400 with relatively low static noise. Considerably better than my 80/40 meter trap dipole.

After that experience I just had to try a bazooka on 75 meters. I used a 100 foot length of RG8X for the two 1/4 wave stubs and added 12 feet of insulated wire to each leg to get the dipole to resonate in the 75 meter band. The result was a 2:1 SWR bandwidth from 3.600 to 4.040 and the same relative quiet I experienced with the 40 meter bazooka.

Each antenna was close to 300khz bandwidth with the 40 meter version being somewhat better and each displayed a static noise level reminiscent of a loop. Both bazookas and the trap dipole are up on the tower installed as inverted vees. The 80 meter versions are at 45 feet apex and the 40 meter bazooka is at the 30foot level. The trap dipole runs SW to NE. The 40 meter runs E to W. The 75 meter bazooka runs NW to SE. They are not all at 90 degrees to eachother but I did not want them to all line up in the same direction.

Antenna selection is through a homebrew antenna switch which grounds the unused antennas. Only one antenna is allowed to go active at a time. I am running power levels up to 1000 watts. That is 1kw output to the antenna.

The dipoles share space with a KT-34M2 beam, a 40 meter loop, and a 20 meter j-pole.

Spam

Desperate times call for desperate measures. Spammers are in desperate times. It appears no one wants to buy their stuff. Now most intelligent people would recognize that fact and try to find other stuff to sell. Stuff that people might want to buy. Spammers don’t understand that and believe that by making people mad folks will buy stuff they don’t need or want.

Does that sound like the reasoning of a sane person? It does not sound sane to me. I would rather not deal with insane people even if they don’t make me mad or are selling stuff I need.

It is hard to shake a bad reputation. Once you step in it the smell lingers on even after you wash up.

Antenna Switches

While it is possible to have one antenna for all bands I find single band antennas are better, more convenient performers. Their main problem is changing out feedlines when changing bands. Well, we use band switches on our equipment, why not use them to switch antennas too?

A decent antenna switch does not need to be any more heavy duty that the bandswitch in your amp or transmitter. It does not need to be coaxial. Short runs of coax braid used for hook-up material inside a metal box is more than adequate and will not adversely effect antenna matching.

Light to heavy duty ceramic switches are adequate for handling powers up to the legal limit in the USofA. Even with 1000 watts of RF we are only experiencing 4.5 amps through the contacts and 220 volts across the contacts when we are dealing with 50 ohm transmission line.

This assumes we are feeding an antenna that is matched to 50 ohms at the transmitter end as well as at the antenna end. The SWR needs to be 1.5:1 or less at all points on the transmission line from the transmitter to the switch to the antenna.

Antenna tuners and transmatches may end up destroying the antenna switch depending on how badly is the mismatch the tuner is compensating. Tuners will match to the transmitter only. They will not match to the feedline and antenna where the antenna switch is located.

So, antenna switches are best applied to systems that use 50 ohm resonant antennas and feedlines.

This may not be common knowledge. Suspected improper use of antenna switches ends up killing the used market for these devices. The boxes may still have use but the welded switch contacts may need replacing.